Hi Stefan,
Check out http://www.aade.com/filter.htm for a good & free filter design
tool. It does support coupled resonator band-pass filters also.
Coupled Resonator band-pass filters are likely to be the best match for your
requirement at the expense of high insertion loss. I don't think you'll get
better with Cauer or elliptic filters where you have to take the inductor's Q
into account at design time.
You could also add another resonator at the antenna which will provide you
with some extra gain and filtering. In that case have the greatest L/C ratio
as possible and extend the length of the whip to 1 meter . This arrangement
is sometimes referred as 'tuned whip'.
Good luck,
Michel - F5WK
Le jeudi 5 août 2010 à 15:32:45, vous écriviez :
> Dear LF,
> Currently i am setting up my active antenna for the planned LF grabber
> here in Heidelberg. It is an active E field antenna, using a BF981 and a
> 125 kHz signal that transforms the 137 kHz down to 12 kHz where some
> band filtering has to be applied. Then, i need another amp stage to
> drive the soundcards input (BF862). The high impedance of the wire input
> is first down transformed by a BF862 stage as a source follower, then i
> allpy a double LF bandpassfilter that is coupled by a C of some pF
> (about 4...8 pF). This signal is applied to the 2nd Gate of the BF981...
> My question is: There may be better suited filter designs than taking a
> L parallel C resonated at 12 kHz (after the mixing stage), between
> signal and ground since this gives a sharp filter, ie 137,7 kHz is
> already attenuated by 25 dB compared to 137,0 kHz. What i want to have
> is a filter with a specific bandwith and edge frequencies with about
> constant low attenuation in the transmission range and relative sharp
> slopes so that 137,7 kHz is not really attenuated but 138,83 kHz
> (DCF-39) as much as possible. DCF39 is 60 dB above noise here although
> it gets already attenuated by the input band filter!
> Jim/M0BMU has designed a filter for his VLF loop RX that looks quite
> good. Is there a web page where i just can type the filter oder, edge
> frequencies, input- output impedances and so on and get the values?
> I have found such one at
> http://www-users.cs.york.ac.uk/~fisher/cgi-bin/lcfilter but i am not yet
> experienced too much about this stuff so i am not sure if this gives
> really useful answers.
> Any other simple ideas to come quickly to the optimal filter type, oder
> and values? I do not want to spend too much time for that, so an
> "excellent filter design book" is not the best hint
> What about a cauer filter? I have read that it has the sharpest edges
> but this may cause QRM in the pass band? (like clicks in a too sharp CW
> filter?)
> The picture shows what i have done so far. Watching the spectrum on the
> roof of the institute (the future QTH) from 0...48 kHz in SpecLab looks
> very promising so far (see picture). DLF is 60 dB above noise although
> already attenuated about 60 dB! So filtering before mixing and further
> amplification is necessary in my case, i assume...
> Tnx for helping ideas.
> 73, Stefan/DK7FC
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